Phosphinidene

Mathey et al. have described an unusual [5C+1S] process involving the reaction of a transient terminal phosphinidene complex [PhP=W(CO)5] with a bu-tadienyl carbene complex yielding a 1-phenyl-1,2-dihydrophosphine P-W(CO)5 complex [112]. 

Keywords. Phosphinidene, P chemistry. Transition metal. Reactive intermediate, Fischer,... 

Phosphinidenes [1] are low-valent organophosphorus compounds that have attracted attention since the early 1980s when they were first discovered [2]. They are known in two classifications, one being the six-electron singly substituted phosphorus species (A) and the other in which the phosphorus atom carries an additional ri -stabifizing group, typically, but not necessarily, a transition metal group (B). Much has been learned about the reactivities of the complexed phos-... 

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

Phosphinidenes have either a singlet ground state with two lone pairs and an empty p-orbital on the phosphorus atom or a triplet ground state in which the phosphorus has instead one lone pair and two singly occupied p-orbitals. Not surprisingly, the electronic preference, i.e., the singlet-triplet energy gap (AEst) and thus the stabUity and reactivity of a phosphinidene, is determined by its substituent. 

Stabilization of the singlet ground state of phosphinidenes can be induced by proper substitution. Such an approach has been applied to carbenes with great... 

Table 1. Bond lengths (A) and singlet-triplet energy differences (in kcal/mol) for triplet phosphinidenes ...

Potentially, phosphaalkenes can be precursors to phosphinidenes in the same manner that carbenes can be formed from alkenes. This latter metathesis route deserves more attention in light of the recognition that stable carbenes can be in equilibrium with their dimers [11]. However, a discussion on phosphaalkenes is outside the scope of the present survey. 

Adding a transition metal group to the phosphinidene in a q -fashion gives singlet species that are more palatable for use in chemical syntheses. Theoretical studies substantiated their preferred singlet nature, the extent of which depends on the substituent on phosphorus. When the transition metal group is M(CO)n... 

Table 2. Singlet-triplet energy differences (A sx) and singlet (BDEj), triplet (BDEx), and composite (BDEsx/xs) bond dissociation energies for phosphinidenes and their W(CO)5 complexes, calculated with BP/TZP...

These composite BDE values increase strongly on substituting the phosphorus atom with n-electron donating groups. However, this substitution effect does not relate to strengthening of the M=P bond,but rather reflects a reduction in the AEji of tho froo phosphinidene or conversely an increase in the AEji for the complexed phosphinidene. Still, as a point of reference, it is noted that similar CASPT2 and BP values of 41.1 and 40.5 kcal/mol, respectively, have been deduced for the dissociation of the parent Cr complex (OC)5Cr=PH into CrfCO) and PH. 

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

The most widely employed electrophilic phosphinidene complexes are generated from M(C0)5 (M=W, Mo, Cr) complexed 7-phosphanorbornadienes 7, which were reported by Marinetti et al. in 1982 [46]. They trapped the transient phosphinidene complex 8 at 110 °C (or at 55 °C if CuCl was used as catalyst) with olefins and alkynes to obtain three-membered phosphiranes and phosphirenes, respectively. 

Cheletropic elimination of the in situ generated phosphinidene complex 8 from the 7-phosphanorbornadiene precursor is believed to be the rate-determining step before 1,2-cycloaddition occurs to the unsaturated hydrocarbon. Without catalysts these are first-order processes that depend only on the concentration of the precursor and not on that of any substrate [47]. The configu-... 

Amino-substituted phosphirane complexes undergo retro-cycloaddition at 70-90 °C to give the trappable (OC)5WPNEt2 phosphinidene complex [55]. Other substituted phosphiranes are not suited to generate phosphinidenes because they require elevated temperatures, which typically induce thermal degradation instead. 

Complexed azaphosphirenes have been used by the Streubel group to generate at 45-75 C in situ phosphinidene complex (OC)5W=PCH(SiMe3)2 (10) together with nitrilium ylid 11 [56]. 

Recently, the group of Lammertsma developed an alternative route, using Collman s reagent and dichlorophosphine 17, to generate Fe(CO)4 complexed phosphinidene 18 in situ [61]. This reactive amino substituted species is trappable at about 0 °C with alkynes and terminal alkenes to give stable phosphirene... 

Table 4. M=P bond distances, M=P-R bond angles, and P NMR chemical shifts of stable cationic terminal phosphinidene complexes ...

The first report on an iron complexed phosphinidene appeared in 1984 by Bertrand and coworkers [63]. They reported that abstraction of Cl" from 22 with PhjC+PFg at -90 °C presumably results in cationic phosphinidene complex 23, which was based on its downfield P NMR resonance at 954 ppm. Above -68 °C the phosphorus center inserts into a isopropyl C-H bond to give 24 as only isolable product. Dehalogenation with AICI3 resulted directly in the AICI4 salt of 24. 

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